Conditional compilation - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include the header file only if DEBUG is defined.

C

#ifdef [1]
#include <stdio.h>
#endif

Drag options to blanks, or click blank then click option'

ARELEASE

BTEST

CLOGGING

DDEBUG

Attempts:

3 left

💡 Hint

Common Mistakes

Using #ifndef instead of #ifdef

Using a macro name that is not defined

Forgetting to close the #ifdef block with #endif

The #ifdef directive checks if DEBUG is defined before including the code inside.

2fill in blank

medium

Complete the code to define a macro only if it is not already defined.

C

#ifndef [1]
#define [1] 100
#endif

Drag options to blanks, or click blank then click option'

AMAX_SIZE

BMIN_SIZE

CBUFFER_LEN

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Defining the macro without checking if it exists

Using a macro name that is unrelated to size

Forgetting to use #endif

The #ifndef directive checks if MAX_SIZE is not defined before defining it.

3fill in blank

hard

Fix the error in the conditional compilation directive to check if VERSION is 2.

C

#if [1] == 2
printf("Version 2\n");
#endif

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A"VERSION"

BVERSION == 2

CVERSION

Ddefined(VERSION)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'VERSION == 2' inside the #if directive incorrectly

Putting quotes around VERSION

Using defined() when comparing values

Use the macro name VERSION directly in #if to compare its value to 2.

4fill in blank

hard

Fill both blanks to compile code only if FEATURE_X is defined and FEATURE_Y is not defined.

C

#if defined([1]) && !defined([2])
printf("Feature X enabled, Feature Y disabled\n");
#endif

Drag options to blanks, or click blank then click option'

AFEATURE_X

BFEATURE_Y

CDEBUG

DRELEASE

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the feature names in the conditions

Using defined() incorrectly

Forgetting the negation operator !

Use defined(FEATURE_X) and !defined(FEATURE_Y) to check the conditions correctly.

5fill in blank

hard

Fill all three blanks to define a macro VALUE as 10 if MODE is 1, else 20.

C

#if [1] == 1
#define VALUE [2]
#else
#define VALUE [3]
#endif

Drag options to blanks, or click blank then click option'

AMODE

B10

C20

DVALUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using VALUE instead of MODE in the condition

Swapping the values 10 and 20

Forgetting to use #define

Check if MODE equals 1, then define VALUE as 10, else define it as 20.

Practice

1. What does the #ifdef directive do in C conditional compilation?

easy

A. Includes the code block only if the specified macro is defined.

B. Includes the code block only if the specified macro is not defined.

C. Always includes the code block regardless of any macro.

D. Excludes the code block if the specified macro is defined.

Solution

1. Step 1: Understand #ifdef usage

   The #ifdef directive checks if a macro is defined.

2. Step 2: Effect on code inclusion

   If the macro is defined, the code inside the #ifdef block is included during compilation.

3. Final Answer:

   Includes the code block only if the specified macro is defined. -> Option A

4. Quick Check:

   #ifdef means "if defined" [OK]

Hint: Remember: #ifdef means "if defined" [OK]

Common Mistakes:

• Confusing #ifdef with #ifndef
• Thinking it always includes code
• Assuming it excludes code if macro is defined

2. Which of the following is the correct syntax to check if a macro DEBUG is NOT defined?

easy

A. #ifdef DEBUG

B. #ifndef DEBUG

C. #if DEBUG

D. #if !DEBUG

Solution

1. Step 1: Identify the directive for 'not defined'

   The directive #ifndef means "if not defined".

2. Step 2: Match with macro DEBUG

   Using #ifndef DEBUG checks if DEBUG is not defined.

3. Final Answer:

   #ifndef DEBUG -> Option B

4. Quick Check:

   #ifndef means "if not defined" [OK]

Hint: Use #ifndef to check if macro is NOT defined [OK]

Common Mistakes:

• Using #ifdef instead of #ifndef
• Using #if !DEBUG which is invalid for macro existence
• Confusing #if with #ifndef

3. What will be the output of this code if FEATURE is defined?

#include <stdio.h>
#define FEATURE

int main() {
#ifdef FEATURE
    printf("Feature is enabled\n");
#else
    printf("Feature is disabled\n");
#endif
    return 0;
}

medium

A. Feature is enabled

B. Feature is disabled

C. Compilation error

D. No output

Solution

1. Step 1: Check if FEATURE is defined

   The code has #define FEATURE, so FEATURE is defined.

2. Step 2: Evaluate #ifdef FEATURE block

   Since FEATURE is defined, the code inside #ifdef FEATURE runs, printing "Feature is enabled".

3. Final Answer:

   Feature is enabled -> Option A

4. Quick Check:

   Defined macro triggers #ifdef block [OK]

Hint: If macro defined, #ifdef block runs [OK]

Common Mistakes:

• Ignoring the #define line
• Thinking #else block runs
• Assuming compilation error due to macro

4. Identify the error in this code snippet:

#include <stdio.h>

int main() {
#ifdef DEBUG
    printf("Debug mode\n");
#else
    printf("Release mode\n");
#endif
    return 0;
}

medium

A. printf statements are invalid

B. Incorrect use of #ifdef directive

C. Missing #define DEBUG before #ifdef

D. Missing closing brace for main function

Solution

1. Step 1: Check code structure

   The code opens int main() { but does not have a closing } brace.

2. Step 2: Verify directives and syntax

   The #ifdef and #else directives are correctly used; missing #define DEBUG is not an error, just changes output.

3. Final Answer:

   Missing closing brace for main function -> Option D

4. Quick Check:

   Every function needs matching braces [OK]

Hint: Check all braces match in code blocks [OK]

Common Mistakes:

• Assuming missing #define is an error
• Thinking #ifdef usage is wrong
• Ignoring missing closing brace

5. You want to compile a program that prints "Debug mode" only if the macro DEBUG is defined, otherwise it prints "Normal mode". Which code snippet correctly achieves this using conditional compilation?

hard

A. #if !DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

B. #ifndef DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

C. #ifdef DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

D. #if DEBUG
printf("Debug mode\n");
#else
printf("Normal mode\n");
#endif

Solution

1. Step 1: Understand the requirement

   Print "Debug mode" only if DEBUG is defined, else print "Normal mode".

2. Step 2: Choose correct directive

   #ifdef DEBUG checks if DEBUG is defined and includes the first print statement; #else handles the other case.

3. Final Answer:

   #ifdef DEBUG printf("Debug mode\n"); #else printf("Normal mode\n"); #endif -> Option C

4. Quick Check:

   Use #ifdef for defined macro check [OK]

Hint: Use #ifdef for defined, #else for else [OK]

Common Mistakes:

• Using #ifndef which reverses logic
• Using #if DEBUG without macro value
• Using #if !DEBUG which is invalid